Organic semiconductor materials may be categorized into the P type and the N type. The P-type organic semiconductor materials, such as pentacene and oligothiophene, are maturer and more stable. Because of cheapness and flexibility, organic materials have been widely used in electronic elements and opto-electronic industry, such as OLED (Organic Light-Emitting Diode), OTFT (Organic Thin Film Transistor) and OFET (organic field-effect transistor). OTFT and OFET are extensively used in the solar cell industry and the flexible electronic industry. In contrast to the traditional inorganic MOS (Metal-Oxide-Semiconductor) elements, the organic semiconductor elements use organic materials to replace the traditional inorganic silicon semiconductor materials to reduce the cost and achieve a slim and flexible element.
The primary electric parameters of FET include electron mobility (μ), on/off current ratio (Ion/Ioff) and threshold voltage (VT). The higher the electron mobility, the rapider the charge movement, and the faster the processing speed of the element signal. The higher the on/off current ratio is, the smaller the leakage current, and the less the power consumption. The lower the threshold voltage, the lower the voltage to drive the transistor element, and the lower the power. Organic molecules are combined to form a crystallization by a weaker force, such as hydrogen bond or van der Waals force. The overlap of the orbits of molecules is smaller. Electrons are transferred via hopping between the orbits or energy levels of molecules.
In an N-type organic semiconductor material, electron is the transferring medium. Thus, the way that molecules stacked in space is particularly important. If a molecular in the crystal has an orientation or if the molecules are stacked orderly along the direction of electron conduction, the barrier of electron hopping is decreased, and higher electron mobility is achieved. N-type organic material molecular carrying negative charge is likely to react with mist and oxygen in air and denatures. Therefore, N-type organic semiconductor materials generally have poor air-stability. After a long time of use, the electron mobility of an N-type organic semiconductor material will be decreased, and even the element itself will fail. In OFET, most metal layers are made of metals having a higher work function, such as gold (4.6 eV) and silver (4.8 eV). If an N-type organic semiconductor material has too low a work function, electrons are hard to transfer from a metal layer to the organic material layer. Therefore, LUMO (Lowest Unoccupied Molecular Orbital) of an N-type organic semiconductor material must be appropriately controlled to match the Fermi levels of the metals of the source electrode and drain electrode to prevent from a Schottky barrier therebetween and favor electron injection.
Organic semiconductor materials are traditionally fabricated with a thermal evaporation method or a vapor phase deposition method. The thermal evaporation method can fabricate an organic semiconductor material with the molecules arranged more orderly to achieve higher electron mobility. However, the thermal evaporation method needs a vacuum environment, which conflicts with the requirement of reducing the fabrication cost. Further, the organic semiconductor materials made of the thermal evaporation method also have a problem of poor air-stability. An organic semiconductor material is usually applied to a flexible substrate. The softening or cracking temperature of a flexible substrate is about 200° C. Therefore, the organic semiconductor material should be fabricated with a low-temperature method, such as the spin-coating method, the inkjet-printing method or the all-soluble method, to overcome the temperature limitation of a flexible substrate.
U.S. Pat. Nos. 7,026,643, 7,198,977, 6,806,368 and 7,326,956 respectively discloses an “Organic N-Channel Semiconductor Device of N,N′ 3,4,9,10 Perylene Tetracarboxylic Diimide”, “N, N′-di(phenylalky)-Substituted Perylene-Based Tetracarboxylic Diimide Compounds as N-type Semiconductor Materials for Thin Film Transistors”, “Liquid Crystalline 3,4:9,10-Perylenetetacarbocylic Acid Diimides”, and “Fluorine-Containing N,N′-Diaryl Perylene-Based Tetracarboxylic Diimide Compounds as N-Type Semiconductor Materials for Thin Film Transistors”. The abovementioned patents respectively disclose different N-type semiconductor materials. However, all the abovementioned N-type semiconductor materials must be fabricated with the thermal evaporation method in a vacuum environment. Further, they still have the problem of poor air-stability.